Yes if the manager fails, then the next manager brings in their ideal players who are suited to their system. This is what happens at every club across the world. That's like saying what's the point of Allowing a manager to sack former staff members to bring in their own only for the next manager to have to do the same.
I agree Garnacho can be developed into a good player but what is the point if he doesn't fit into the Manager's plans and is forced on him? It can also be more about ability, but also character, we know Amorim likes to assess not just ability but personal individual traits, and maybe just maybe Garnacho's character is not up to the standard requirements of Amorim, hence why both him and Rasford were dropped and now up for sell.
The new board who also have vested interest in ensuring the benefits of the club is run professionally long term, and so maybe they are fully on board with the decision to sell considering Ratcliff was not amused when Garnacho brought that Ispeed clown to the fa cup after party and it's well known that his brother causing all sorts of drama on social media. He can be a good player or potentially a bad egg in the future, we don't know and they've probably weighed up the decision that gamble is not worth taking consideriing he is not going to turn out to be an elite talent, that much we do know.
The club is bigger than one individual player and I feel far too many people have grown far too attached to this idea that we must keep every academy player
In regards to structure, having a structure doesn't mean you disregard the Manager's requirements, the structure is to set out guidelines and objectives for the manager and to assist him the best possible way to help them carry out their role, whether it be transfers, training facilities and organising to help them bring in their own staff.